Evidence of diffusive fractal aggregation of TiO₂ nanoparticles by femtosecond laser ablation at ambient conditions

Abstract

The specific mechanisms which leads to the formation of fractal nanostructures by pulsed laser deposition remain elusive despite intense research efforts, motivated mainly by the technological interest in obtaining tailored nanostructures with simple and scalable production methods. Here we focus on fractal nanostructures of titanium dioxide, TiO 2 , a strategic material for many applications, obtained by femtosecond laser ablation at ambient conditions. We model the fractal formation through extensive Monte Carlo simulations based on a set of minimal assumptions: irreversible sticking and size independent diffusion. Our model is able to reproduce the fractal dimensions and the area distributions of the nanostructures ob- Introduction.

Growth dynamics of pulsed laser deposited nanofoams

“…We also note that the aggregation process does not seem to be affected by the chemical affinity between substrate and nanoparticles (as in the case of aC substrate) or by the presence of droplets on the Rh surface that could have acted as nucleation seeds if diffusion-limited aggregation on substrate were the dominant aggregation mechanism at play. We note that this result differs from the surface diffusionlimited aggregation model proposed in [46,47] to explain the aggregation of gold and titania nanoparticles in femtosecond PLD experiments. On the other hand, it substantiates the claim put forward in [44], where the fractal structure obtained by subpicosecond PLD of iron is attributed to an in-flight clustercluster aggregation process.…”

“…with diffusion-limited aggregation mechanisms occurring on the substrate surface, as those observed in [46,47].…”

“…In contrast with the rich and vast body of literature describing the synthesis of nanoparticles in laser ablation experiments, the physics of how the nanoparticles spatially arrange themselves to give rise to the peculiar nanofoam morphology, i.e., the physics of nanoparticle aggregation which determines the growth of nanofoam films, is poorly understood. Only a few articles are devoted to the study of fractal structures resulting from the coalescence of laser-ablated nanoparticles with various elemental compositions [44][45][46][47]. In these works, two main aggregation mechanisms are proposed to explain the morphological features of the deposited structures, namely nanoparticle diffusion on substrate [46,47] and nanoparticle aggregation occurring in flight [44].…”

“…Only a few articles are devoted to the study of fractal structures resulting from the coalescence of laser-ablated nanoparticles with various elemental compositions [44][45][46][47]. In these works, two main aggregation mechanisms are proposed to explain the morphological features of the deposited structures, namely nanoparticle diffusion on substrate [46,47] and nanoparticle aggregation occurring in flight [44]. However, a complete and satisfactory picture of the various processes (and the corresponding timescales) that take place during the PLD process is still lacking, and many questions regarding the relation between material properties (e.g., average density and average aggregate size) and PLD parameters are not answered yet.…”

Coalescence of Cluster Beam Generated Sub‐2 nm Bare Au Nanoparticles and Analysis of Au Film Growth Parameters

“…Regarding the film growth characteristics of cluster beam generated NPs and how NPs on the substrate interact with each other the literature is still scarce. An interesting example is represented by the work of Celardo et al . who used femtosecond laser ablation deposition in air at ambient conditions to study the fractal nanostructures of titanium dioxide, TiO2, on a variety of substrates.…”